Since their accidental introduction to Taiwan in 2003 the red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae) have become an important economic, agricultural, as well as a local biodiversity and public health problem in both rural and urban areas. Several techniques are being used around the world either to control the fire ant population or to limit their further expansion. To control S. invicta over a larger area, toxic bait products are the most effective tool (Vogt et al., Environ. Entomol. 32: 447-451, 2003).
The ant baits are prepared with three major components, a toxicant, a vegetable oil as a phagostimulant, and a carrier (usually corn grits) for the phagostimulant/toxicant solution to allow easy distribution of the bait and to let fire ants carry baits back to their nests to feed larvae and gyne to reach the purpose of killing gyne. The bait should be fresh and tasty so that it can attract fire ants to carry it into their nests. However, the drawback of commercial bait is that when it exposes to a humid place, it is easily to break down. Moreover, after keeping commercial bait in water for 0.5 hour, it becomes attached to each other forming a single soft ball, and the chemical properties of the vegetable oil used as an attractant could be destroyed. Therefore, there is a strong market demand for the development of a high humidity or water resistant ant bait that can make the viability of ant bait less dependent upon good weather, even can be used when raining days. The humid resistant ant bait is especially useful at humid places such as Taiwan, Hong Kong or mainland China, because in these areas, basically there has not many days to carry out the fire ant control programs determined by it does not rain. Therefore, developing bait for humid regions plays an important role in control fire ants (Kafle et al., Sociobiology 53: 729-737, 2009).
Some attempts have been made to develop moisture resistant ant bait by coating the corn grit carrier by microencapsulation using existing coating material (Kafle et al., Sociobiology 53: 729-737, 2009) or by finding a new coating material or new carrier. However, Kafle et al. (Kafle et al., Sociobiology 53: 729-737, 2009) reported that although microencapsulated corn grit bait could resist water, the ants were not stimulated to feed on them. U.S. Pat. No. 4,874,611 discloses a microencapsulated ant bait. It uses a gelling agent such as carrageenan and the attractant were added to the shell. Even though it can also be water-resistant, but the cost is high, and the producing is inconvenient. Thus, using microencapsulation may not be a suitable alternative towards the development of a high humidity or water resistance fire ant bait.
In addition to bait development and improvement, TW patent number I262763 discloses an ant feeding device, comprising a tube body and a combination piece, wherein the tube body has a capacity to place bait, and the capacity has a hole. The combination piece combines the tube body, and has a block piece corresponding to the hole in order to block or partial block the hole to supply the bait quantitatively. The combination piece can be inserted into ground, making the hole maintain appropriate distance from the ground to avoid the bait getting wet.
There are also several methods and devices to eradicate fire ants at present. One of them is by inserting a tip of the lance assembly into a Solenopsis infested mound and then delivering vaporized insecticides in to the mound by the lance to eradicate fire ants (U.S. Pat. No. 4,756,118). Another method is to drench the whole fire ant mound with water and insecticide mixture (U.S. Pat. No. 5,054,231). The above two methods all need to contact drugs or aqueous solution containing insecticides with gyne and fire ants to achieve the desired effect. In particular, some fire ants bury the gyne into the ground below one meter, and it is difficult to let the drugs or aqueous solution containing insecticides to go into such a deep place, and this method has the risk of environmental pollution.
Another way to kill fire ants is by using a tent covering the ant mound and injecting vapor gas to the ant mound (U.S. Pat. No. 5,031,355). Recent development is to use a probe injecting hot steam into ant mound to kill fire ants (U.S. Pat. No. 6,026,609). The above two methods are major in using the heat steam to kill fire ants. The disadvantage is that when the steam jets into ant mound, it would condense into drops and block the access. The follow-up steam can not be sustained jet into the bottom of the ant mound so that it can not eradicate fire ants completely. It needs using this method several times to achieve the desired effect. Using refrigerant or coolant such as liquid nitrogen is also a way to kill fire ants (U.S. Pat. No. 5,165,199). Using liquid nitrogen to kill fire ants is by its low-temperature −196° C. to freeze fire ants. Although the effect of this method is good, the low temperature of liquid nitrogen is not easy to carry and operate, and it requires specific equipment of thermal insulation tank and pipe to store and transfer the liquid nitrogen before injecting into the fire ant mound. Because the complicating equipment, cumbersome operation, high-cost of liquid nitrogen and the inconvenience of economy and use, making universal use of this method difficult. It had also used hot air spraying on indoor carpet to eradicate insects and other tiny organisms. This method is effective for insects and other tiny organisms which expose to outside, but it is useless for red fire ants which are deep in the underground. Taiwan Patent No. I264286 discloses fire ant eradication device using high-pressure heated gas pulses and method thereof. The device and method for killing fire ants are by introducing of high-pressure hot gas pulses to the fire ant mound. A metal hollow probe having a plurality of orifice near its bottom is mechanically or manually inserted into fire ant mound. Hot gas in the metal hollow probe is ejected in pulse into the ant mound from the orifices, through channels in the ant mound, whereby hot gas is filled in the whole ant mound rapidly, heating and killing the fire ants and ant queens before they move to safety. Although the above mentioned methods can eradicate fire ants, the equipments and materials of these methods are not available on the market, and the operation may be complicated and dangerous.